This is a tough & complex situation, but the fact that your daughter ended up confiding in both you and your mother is a good sign that you can reach her in a positive way. I don't think it is wrong to be your daughter's confidant and not include your husband in this particular aspect of her life - at least not until you find out if there is an underlying cause to the promiscuity.
I was raised in a Christian home by two strict, but loving parents, attended church every Sunday and went to private school. I too did very well in school. But I knew what I could and could not discuss at home, so I was selective in only bringing up things that would not cause any discourse. In retrospect, I wish I had a confidant about ALL matters of my life at that age.
I lost my virginity in HS to someone I didn't love (I guess I just wanted to tell friends that I had done it too). That opened the floodgates and suddenly I was promiscuous with almost every guy I went on a date with - it was a downhill spiral that lasted a few years that included episodes of contracting an STD (curable, thank goodness). My parents never found out because I would get the issues taken care of at free clinics.
I guess the male attention gave me a temporary feeling of self-worth and acceptance. The weird thing was, I always felt sick to my stomach with guilt after having sex and then felt completely filthy, but it was not enough to stop doing what I was doing at that time.
I appeared to have a lot going for me on the outside, but I was lacking something on the inside. Your daughter will need help to figure out what she is missing, but you've already taken a good step with the therapy. One suggestion, make sure your husband is doing everything he can to praise your daughter when necessary and to show his acceptance of her. It could help a lot.
Since you did what you could to prevent her from having sex too early, but it happened anyway, she'll now need your moral guidance regarding monogamy and weighing out consequences of her actions. Emphasize the importance of being selective and why it is best to wait until Mr. Right comes along. She has already gotten just a small taste of what a Mr. Wrong can do.
Emphasize everything that she has to offer a guy. If all he likes about her is the sex, then he is not worth her time. Among other complications, there is more emotional bad that will come out of it then the temporary good.
Discuss the importance of condoms and maybe even consider allowing her to go on birth control even once she finds Mr. Right. I know this sounds like you're condoning, but frankly, now that she has started, she will most likely continue to do it with or without your knowledge, so you will want to offer the best usable advice and guidance she is willing to take. Let her know that she can discuss ANYTHING with you and you will never stop loving her.
Ultimately, you should tell her that you may not have an agreeable opinion, but you are offering sound advice so that she can choose to do with it what she wants.
Best of luck and my prayers are with you.